This paper examines the long-run effects of the 1906 San Francisco Earthquake on the spatial distribution of economic activity in the American West. Using variation in the potential damage intensity of the earthquake, we show that more severely affected cities experienced lower population increases relative to less affected cities until the late 20th century. The earthquake left a long-lasting mark mainly because it interrupted existing migrant networks. Less affected areas became more attractive migrant destinations in the immediate aftermath of the earthquake, which permanently changed relative city sizes in the American West.